Electric motor comprising a stator cooling unit

ABSTRACT

A fixed stator is arranged around a rotatably mounted rotor in an electric motor that includes at least one cooling unit to which parts of the stator which are to be cooled are thermally coupled by a line system in which a cooling agent circulates according to a thermosyphon effect. The stator parts to be cooled can be arranged in the inner region of a stator housing which is integrated into the line system. The electric motor can be provided with a heating device to maintain the pressure in the inner region when the motor is stopped.

This application is based on and hereby claims priority to GermanApplication No. PCT/DE03/01705 filed on May 26, 2003 and German PatentApplications 102252224.6 filed Jun. 6, 2002 and 10317967.4 filed Apr.17, 2003, the contents of all of which are hereby incorporated byreference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The invention relates to an electrical machine having] a rotor which ismounted such that it can rotate,] an associated, stationary stator, and]a device for cooling at least the stator or parts of it.

2. Description of the Related Art

A corresponding machine is disclosed in EP 0 853 370 A1.

A considerable amount of heat may be developed in the stator of machinesor motors, particularly with relatively high power levels, and this hasto be dissipated by cooling measures in order to achieve higher machineefficiency. By way of example, air-cooled generators (in particular withratings below 300 MVA) are known, in which cooling is achieved by acomparatively large air flow which is passed through a network of finerchannels (see the EP-A1 document cited initially). In this case,however, the air flow itself contributes to undesirable heat beingproduced to a considerable extent, as a consequence of friction lossesin the channels.

For relatively large machines such as generators, it is also known forthe stator and rotor to be cooled with hydrogen gas (see, for example“Proceedings of the American Power Conference”, Volume 39, Chicago 1977,pages 255 to 269), which is circulated in an encapsulated housing. Inthis case, not only are complex sealing measures required, but extensivesafety measures also have to be taken into account.

Furthermore, water-cooled generators are also standard, in which thewater is circulated in channels which, in particular, extend through theso-called stator bars (and laminated stator cores). The use of pumps isnecessary for this purpose.

Furthermore, the water must be conditioned, for corrosion protectivereasons.

SUMMARY OF THE INVENTION

An object of the present invention is therefore to refine the machinewith the features mentioned initially so as to allow effective coolingwith relatively little complexity.

According to the invention, this object is achieved by the coolingdevice for the machine having at least one cold surface of arefrigeration unit to which the parts of the stator to be cooled arethermally coupled via a line system, in which a circulation of a coolantis provided or is carried out on the basis of a thermosiphon effect.

A line system such as this has at least one closed pipeline, which runsbetween the cold surface of a refrigeration unit and the parts of thestator to be cooled, with a gradient. The coolant which is located inthis line system in this case recondenses on the cold surface of therefrigeration unit, and is passed from there into the area of the statorparts to be cooled, where it is heated and, in the process, generallyvaporized. The coolant, which is thus generally vaporized, then flowswithin the line system back again into the area of the cold surface ofthe refrigeration unit. The corresponding circulation of the coolantaccordingly takes place on the basis of a so-called “thermosiphoneffect” in a natural circulation with boiling and vaporization. Thus,according to the invention, this principle which is known per se isapplied to the cooling of stator parts of power electrical machines.

In comparison to air-cooled machines, this allows the air volume flow tobe reduced by partial direct heat dissipation at the point where theheat losses are generated, via a thermosiphon. This results in areduction in the development of heat that is produced by the air flow,which allows a further reduction in the air volume flow. This thusresults in higher machine efficiency and savings in production costs, inparticular for the winding and the laminated core of the stator.

If the stator is cooled completely by thermosiphoning, the power limitbeyond which hydrogen cooling is normally used instead of air cooling isshifted to considerably higher power ranges.

In comparison to direct water cooling of stator windings with forcedcirculation, the advantages are as follows:

-   -   No corrosion or complex conditioning of the coolant when using        organic coolants such as butane, propane or acetone.    -   There is no risk of fire or explosion, owing to the use of a        closed line system.    -   Furthermore, the cooling device is maintenance-free, does not        contain any pumps or other moving mechanical parts, and is,        furthermore, self-regulating.

The advantages associated with the refinement of the machine accordingto the invention are thus that the power range from which direct statorcooling is worthwhile can be reduced.

The cold surface can thus be arranged in a simple manner on or in acondenser area, which is integrated in the line system.

Furthermore, at least one coolant area can advantageously be integratedin the line system, in which stator parts to be cooled make a large-areathermally conductive connection with the coolant, between which and thestator parts to be cooled good heat exchange is ensured.

The internal area of a stator housing can particularly advantageously beprovided as a coolant area in which at least the majority of the partsof the stator to be cooled are arranged. This internal area is inconsequence in the form of an integrated part of the thermosiphon linesystem. This is based on the assumption that the majority of the statorparts to be cooled include more than 50% of the volume of the parts ofthe stator which are heated without cooling, in particular such as thewinding and, possibly, laminated cores for carrying the magnetic flux.In this context, a stator housing is the housing which fixes theinternal area with the stator parts to be cooled and with the coolantwhich cools them. The advantages of this refinement of the machine aremainly that the heat-generating parts of the stator are at least largelysubjected to the coolant, as heat exchanging surfaces, thus ensuringcorrespondingly good heat absorption by the coolant.

The stator parts to be cooled in the internal area advantageously make alarge-area thermally conductive connection with the coolant. In thiscase, the stator parts to be cooled may also include laminates of alaminated core, in addition to a stator winding. Since heat is likewiseproduced in laminates such as these during operation, this caneffectively be transferred to the coolant.

Furthermore, the stator of the machine may have cooling channels, whichare integrated in the line system. Cooling channels such as these areparticularly advantageous for the operation of the thermosiphon when thestator is arranged vertically (with the rotor axis running vertically),since any coolant vapor that is then produced can flow away well.

Furthermore, in order to assist the heat dissipation, the cooling devicemay also have flow paths for air cooling.

In addition, it may be regarded as particularly advantageous for aheating apparatus to be provided on or in the line system, in an area inwhich the coolant is at least largely in the liquid state. Specifically,a heating apparatus such as this makes it possible to reduce orcompensate for undesirable pressure differences between the statorinternal area, which is filled with the coolant, and the surroundingoutside area when the machine is stationary (=shutdown in operation).This is because, when the machine is stationary, the stator generatesvirtually none of the heat that results in the heating of the coolant.This means that the internal area of the stator housing is cooled everfurther owing to the cooling power which is introduced via the coolantas before, so that the pressure falls well below the environmentalpressure. In conjunction with low external temperatures and materialshrinkage, such a reduced pressure could result in leaks in the statorhousing, via which air could be sucked in. This would lead to theboiling line of the coolant that is used being shifted, thus in the longtime rendering the thermosiphon circuit ineffective. This risk can beprecluded by using the special heating apparatus. This is because theheating apparatus makes it possible to prevent the stationary pressurefalling below the environmental pressure in the stated area, preferablyin an end-face area of the stator. The supply of heat results in thecoolant being vaporized even when the machine is stationary. Thecorresponding vapor then condenses at cold points in that part of thethermosiphon line system which is formed by the stator internal area,where it thus heats the line system to a largely uniform temperature.This is associated with a pressure rise in the line system,corresponding to the boiling characteristic of the coolant that is used.In this case, the heating power can advantageously be regulated via apressure sensor, so as to set a pressure at least equal to theenvironmental pressure in the line system. Since virtually no powerlosses occur during a shutdown in operation, the heating apparatus hasto compensate only for the convective losses via the stator housing tothe environment.

BRIEF DESCRIPTION OF THE DRAWINGS

These and other objects and advantages of the present invention willbecome more apparent and more readily appreciated from the followingdescription of the preferred embodiments, taken in conjunction with theaccompanying drawings of which:

FIG. 1 is a combined perspective and block diagram showing statorcooling by a vaporizer cooler for a machine,

FIG. 2 is a block diagram showing direct stator cooling by discretecooling channels within a stator housing of the machine,

FIG. 3 is a block diagram showing a further refinement of the machine,with a coolant area in a stator housing, and

FIG. 4 is a graph of the temperature-dependent pressure ratios in thecoolant in the machine shown in FIG. 3.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

Reference will now be made in detail to the preferred embodiments of thepresent invention, examples of which are illustrated in the accompanyingdrawings, wherein like reference numerals refer to like elementsthroughout.

The electrical machine according to the invention is based on machineswhich are known per se in the higher power range, such as generators.Parts which are not illustrated are generally known. Only those parts ofthe machines which are significant to the invention are shown in thefigures.

According to FIG. 1, the machine 2 has a cooled or uncooled rotor 3,which is mounted such that it can rotate about an axis A. The rotor isat least partially surrounded by a stator 5 while maintaining anintermediate space 4 with an annular cross section, of which stator 5 inFIG. 1 illustrates only individual laminates 5 _(i) of a laminated core.A coolant area 7 in the form of a disk is formed between two of theselaminates 5 ₁ and 5 ₂, which are in the form of disks and areillustrated exploded axially in FIG. 1. Corresponding coolant areas areintegrated or stacked and/or pushed in into the laminated core atspecific intervals (seen in the axial direction). This ensures there arelarge heat exchanging surface areas between a coolant k which is locatedin the at least one coolant area, and the adjacent laminates of thelaminated core 5.

Depending on the requirement for the temperature level to be chosen,liquefiable gases such as propane, butane, acetone or neon, orazeotropic mixtures that are used in standard refrigeration technology,may be used as the coolant.

In design terms, the at least one coolant area 7 can be producedadvantageously in the following manner, specifically

-   -   by two laminates which are separated by spacers and are welded        together in a pressure tight manner along the edges,    -   or by the use of elements which are held at a distance from one        another by the introduction of beads.

The at least one coolant area 7 is part of a closed line system 10 forthe coolant k circulating in it. At a geodetically higher level, theline system contains a condenser area 8, which is connected to thecoolant area 7 between the stator laminates 51 and 52 via a coolantsupply line 11 and a coolant return line 12.

The refrigeration power for cooling of the stator is provided by arefrigeration device, which is not illustrated in any more detail butwhich, for example, has at least one cold head located at its cold end.A cold head such as this has a cold surface 14 which is of any desiredshape but must be kept at a predetermined temperature level, or isthermally connected to such a cold surface 14. The internal area of thecondenser chamber 8 and thus the coolant are thermally coupled to thiscold surface; for example, the cold surface 14 may also form a wall ofthis area.

The coolant condenses on the cold surface 14 and, as a result of thegeodetic grading, passes in liquid form (which is annotated k_(f)) viathe supply line 11 into the coolant area 7 in the area of the laminatedstator core 5 to be cooled. The coolant level there is annotated 9.There, the coolant is heated, for example being at least partiallyvaporized, as is intended to be indicated by individual vapor bubbles 9′in FIG. 1. The coolant k_(g) which is thus gaseous, flows out of thisarea 7 via the return line 12 into the condenser area 8, where itrecondenses on the cold surface 14. A natural circulation such as thiswith boiling and vaporization forms the thermosiphon principle (see alsoDE 41 08 981 C2 or DE 100 18 169 A1).

A combination of air cooling with thermosiphon cooling of its stator 25is provided for the electrical machine 22, which is illustrated onlypartially in the form of a section in FIG. 2. In this case, the aircirculates in a known manner (see, for example, EP 0 853 370 A1, whichwas cited in the introduction, or EP 0 522 210 A1), and is illustratedby lines Lf with arrows on them. In addition, cooling channels 27 of aline system 20 run in the axial direction through the core of the statorlaminates 25 _(i). At the ends, these cooling channels once again openinto a coolant supply line 11 and a coolant return line 12. These lines11 and 12 are connected to a condenser area 28 with a cold surface 14for cooling down the coolant which is circulated in the line system 20using a thermosiphon effect and is in general annotated k. The lines 11and 12 either open into this area, in which condensation of gaseouscoolant k_(g) then takes place to form liquid coolant k_(f).Alternatively, as is assumed for the exemplary embodiment, indirectcooling is provided by a further coolant k′, which fills the area 28. Inthis case, the line system 20 runs through this area where heat isexchanged with the coolant k′ through the wall of the line system. Thus,in this embodiment, instead of being subjected to forced circulationcoolant by water, the stator bars and laminates 25 _(i) are in thisembodiment cooled in a closed circuit with a thermodynamicallyadvantageous coolant k, which is matched to the operating state (pT),with the laminates 25 _(i) together with their cooling channels 27 beingused as vaporizers. Owing to the two separate lines 11 and 12, thethermosiphon line system 20 is also referred to as a “two-pipethermosiphon”.

The exemplary embodiments which have been explained with reference tothe figures advantageously use a number of vaporizer coolers which areoptionally either connected by individual cooling circuits to thecondenser area, or whose supply and return lines are in the form ofjoint lines. The advantage in this case is the smaller pipeworkcomplexity, in which case it is necessary for the individual vaporizersto ensure that the coolant flows are split on the basis of the thermalrequirement. Owing to the large amount of heat transferred duringcondensation, the physical volume for cooling down and thus the costsare reduced by the use of the thermosiphon cooling in comparison toair/air cooling or air/water cooling.

In contrast to the provision of the cooling power, as assumed for theembodiments shown in FIGS. 1 and 2, by the cold head of a gryogeneniccooling at a relatively low temperature level, it is possible,particularly when comparatively higher operating temperatures arepermissible, for a coolant to be cooled down on a cold surface by wateror environmental air, as well. This is because the only precondition forcirculation of the corresponding coolant based on the thermosiphoneffect is the temperature gradient between the cold surface of arefrigeration unit and the stator parts to be cooled.

A further exemplary embodiment of a machine according to the inventionwith a particular refinement of the thermosiphon line system for itscooling device is illustrated schematically, in the form of a section,in FIG. 3. In this case, this FIG. 3 essentially shows only theconfiguration of a refrigeration device. The machine, which is annotatedin general 30, contains a stator 31 with a stator housing 32 whichsurrounds an internal area 33, which is sealed on the outside. At leastthe majority of the stator parts to be cooled are intended to be locatedin this internal area. A stator winding 34, which is known per se,together with further stator parts, in particular for retaining orholding the winding, and for guiding the magnetic flux, such aslaminated cores, are accordingly accommodated in the internal area 33.The internal area 33 is advantageously in the form of an integrated partof a thermosiphon line system 35, whose method of operation correspondsto the method of operation of the line system 20 described withreference to FIG. 2. When the machine is in operation, the liquidcoolant k_(f) supplied via the supply line 11 absorbs heat that isproduced by the stator parts to be cooled, and is vaporized in theprocess. In order to improve the dissipation of the vaporized, gaseouscoolant k_(g), particularly if the machine or its axis A is arrangedvertically, cooling channels or pipes 36 may also run through the statorparts to be cooled. In this case, pipes 36 which project above thefilling level are advantageous for a vertical arrangement, as is thebasis of FIG. 3, since vapor which is produced in the lower part of thehousing can be dissipated well upwards via them.

When the machine 30 is stationary, corresponding heat sources arelargely absent. An electrical heating apparatus 38 can thereforeadvantageously be associated with the thermosiphon line system 35 in anarea which the liquid coolant k_(f) coming from a condenser area 28enters. This area 37 may preferably be located on the end face of thestator 31, or possibly also at a point on the coolant supply line 11 atwhich the coolant k_(f) is still in the liquid state. This heatingapparatus allows the coolant to be additionally heated, preferablyvaporized, so that this results in a pressure increase in the internalarea 33, starting from the area 37. This means that this heatingapparatus can be used to regulate the pressure in this area. The heatingpower for setting the pressure is in this case controlled using knowntechniques which may, in particular, include the use of pressuresensors.

One exemplary embodiment of a corresponding pressure increase isindicated in the graph in FIG. 4 for the coolant with the itemdesignation “R236fa”. In this case, the temperature T of the coolant isplotted in the abscissa direction in the area 37 (measured in ° C.), andthe pressure p in the coolant (measured in bar=10⁵ Pa) is plotted in theordinate direction. As can be seen from the graph, the heating apparatus38 according to the invention can be used to produce a pressureincrease/to regulate the pressure at −40° C., the temperature of theliquid coolant k_(f) that is supplied, of, for example, about 0.1 bar toabout 1.0 bar at this temperature. A pressure increase such as this ispreferably planned when the rotor 3 of the machine 30 is stationary andthere is a risk of excessive cooling of the stator 31 with a pressuredrop in its internal area 33. The curve p1 on the graph describes thepressure relationships which would occur in the internal area of thestator without additional heating power from the heating apparatus whenthe rotor is stationary. In this case, the curve p1 represents theboiling line of the chosen coolant. The pressure relationshipsillustrated by the curve p2 are obtained with the heating apparatusswitched on, and allow an increase to the environmental pressure aroundthe stator housing 32 to, for example, 1 bar. In this case, the amountof additionally heating power introduced into the coolant is expedientlyonly as much as is required to compensate for the pressure differencesbetween the internal pressure in the line system and the environmentalpressure.

The heating apparatus according to the invention can also, of course, beused to provide additional heating power during rotation of the rotor,if the heat generation caused in the interior by the stator parts to becooled is not sufficient.

The embodiment of the machine 30 illustrated in FIG. 3 is based on theassumption that the heating apparatus 38 is located exclusively in theend-face area 37 of the stator 31. Arrangement of this heating apparatusin this area is admittedly regarded as particularly advantageous, sinceheating of the coolant, which is generally still liquid when enteringthe stator, takes place in any case there. It is, of course, alsopossible for the heating apparatus to extend—seen in the flow directionof the coolant—from the end-face area into axial areas of the statorinternal area or of the line system as well, if the coolant there isstill in the liquid state. However, if required, the heating apparatus38 may also be fitted to the supply line 11, upstream of the inlet areaof the liquid coolant kf into the stator.

In general, an electrically heated apparatus 38 is provided directly onor in the thermosiphon line system. However, if required, the heatingpower can also be introduced into the coolant in some other manner, forexample indirectly via a heat exchanger.

The invention has been described in detail with particular reference topreferred embodiments thereof and examples, but it will be understoodthat variations and modifications can be effected within the spirit andscope of the invention covered by the claims which may include thephrase “at least one of A, B and C” as an alternative expression thatmeans one or more of A, B and C may be used, contrary to the holding inSuperguide v. DIRECTV, 69 USPQ2d 1865 (Fed. Cir. 2004).

1-10. (canceled)
 11. An electrical machine, comprising: a rotorrotatably mounted; a stator associated with said rotor in a stationaryposition; and a cooling device, cooling at least parts of said stator,including a refrigeration unit having at least one cold surface; and aclosed line system, thermally coupling said refrigeration unit to theparts of said stator to be cooled, having discrete coolant areasassociated with the parts of said stator to be cooled, and in which acoolant is circulated by a thermosiphon effect, the coolant being heatedor at least partially vaporized in the discrete coolant areas.
 12. Themachine as claimed in claim 11, further comprising a condenser areawhere said closed line system is thermally coupled to the cold surfaceof said refrigeration unit.
 13. The machine as claimed in claim 12,wherein the discrete coolant areas are thermally conductively connectedover a large area to the stator parts to be cooled.
 14. The machine asclaimed in claim 13, wherein said stator has a laminated core, andwherein the discrete coolant areas are formed between laminates of thelaminated core of said stator.
 15. The machine as claimed in claim 12,wherein the discrete coolant areas are formed as cooling channels. 16.The machine as claimed in claim 15, further comprising flow paths forair cooling.
 17. The machine as claimed in claim 11, wherein thediscrete coolant areas are thermally conductively connected over a largearea to the stator parts to be cooled.
 18. The machine as claimed inclaim 17, wherein said stator has a laminated core, and wherein thediscrete coolant areas are formed between laminates of the laminatedcore of said stator.
 19. The machine as claimed in claim 11, wherein thediscrete coolant areas are formed as cooling channels.
 20. The machineas claimed in claim 19, further comprising flow paths for air cooling.